In a Computed Radiography (CR) reader, the laser beam is scanned in a scanline over the surface of a storage phosphor screen by a reciprocating galvanometer mirror or polygon rotating mirror in a fast scan direction while the screen is transported under the scanline in a slow scan direction. To make the CR reader compact and manufacturable, a final fold mirror is placed in the optical path between an F-theta lens and the phosphor screen. The fold mirror introduces three degrees of freedom into the optical system. A coordinate system on the mirror has an x-axis in the plane of the mirror along the fast scan dimension of the mirror, a y-axis in the plane of the mirror perpendicular to the x-axis and a z-axis normal to the mirror surface.
The z-axis degree of freedom allows the path length of the optical system to be adjusted by translating the mirror along that axis. Typically, the depth of focus of the laser is not critical and there is no need to adjust this degree of freedom.
The other two degrees of freedom are rotations about the x and y axes which allow the scanline to be positioned on the phosphor surface. It is required to rotate the scanline perpendicular to the slow scan transport direction to eliminate a parallelogram image distortion. It is also required to translate the scanline to a particular position where the laser beam is not obstructed and the phosphor screen is well controlled for height.
The alignment of the scanline for rotation and translation in a known CR reader was done visually by an operator observing the scanline while adjusting the two axes. This required safety hardware and procedures to be followed to ensure laser safety in the presence of the exposed beam. Also the process resulted in significant residual misalignment since the adjustment process was a subjective operator dependent alignment.